Topical fluiride agents currently in use clinically (APF, NaF, SnF2) were developed because of their ability to deposit substantial amounts of fluoride in human enamel. Recent clinical trails employing these conventional agents have failed to demonstrate the levels of caries reduction reported in earlier trails, particularly in flouridated areas where little or no benefit has been found. Most of the fluoride deposited in sound enamel from these agents has been shown to be in the form of CaF2 which is unstable in the oral environment. Several regimens aimed at enhancing the fluoride retention by enamel and the cariostatic effect of topically applied fluorides have been studied in vitro. Three promising approaches include (1) a regimen reported to increase the uptake of fluoride in a stable form consisting of a pretreatment of the enamel with a saturated DCPD solution prior to an application of one of the previously mentioned conventional fluoride agents, (2) a combined application of APF AND SnF2 which has been shown to enhance the resistance of enamel to acid dessolution, and (3) the application of concentrated sodium monofluorophosphate which reportedly enhances remineralization. The objective of this study is to determine whether single and/or multiple applications of these regimens can increase the resistance of sound and "white spot" enamel to artificial caries in vitro. An acidified gelatin gel system will be used to produce artificial caries-like lesions in vitro which are indistinguishable from natural caries when examined by polarized light microscopy. Histological parameters related to the initiation and progression of such lesions will be evaluated in polarized light. The effect of the various regimens on modifying the histological zones of the lesion and their pore volume will also be investigated Quantitatively using "contour maps." In addition, attempts will be made to correlate these findings to physical papameters such as (1) uptake of fluoride in a stable form as determined by an acid-etch biopsy technique. (2) alterations in the enamel surface including the production of surface coating, and (3) changes in the crystal structure of the subsurface enamel as evidenced by scanning electron microscopy. The long-term objectives of this study are to develop more effective and efficient methods of administering topically applied fluorides based upon a better understanding of their mechanisms of action.